Automotive catalytic converters of the pellet (or "bead") type are currently sheet steel structures which sandwich and support a bed of ceramic pellets coated with a catalyst, usually a noble metal. Typically the pellets are contained between a pair of perforated sheet steel retainers which define the bed. The bed is arranged within a container so that hot engine exhaust gases must pass over, down and through the catalyst pellet bed. The gases then exit the converter in a less noxious state.
Many configurations of pellet beds and container housings have been developed and used and numerous improvements have been made, particularly since catalytic converters became essential equipment on automotive engines. However, automotive catalytic converters continue to have significant problems, some of which have led to costly recalls in the automotive industry. Improvements are needed for better functioning and longer, more reliable life in such devices.
Catalytic converters must survive the turbulent hot exhaust stream and complete the combustion of the gases, preferably without adding undue backpressure in the exhaust system. In use, particularly at hot operating temperatures, exhaust flow can agitate, swirl and grind the ceramic pellets to dust. This action in the pellet bed, sometimes referred to as pellet fluidization, is most harmful to operation of the catalytic converter.
The primary approach in current catalytic converter design to retarding pellet fluidization involves supporting the body of pellets ("pellet bed") in a rigid manner. Heavy stainless steel retainers which are pinned by thick steel studs fix the geometry of the bed. However, the thermal cycling and vibration which are inherent in the operation of an automobile provide room for the pellets within the bed to agitate. Over time, voids appear, louvers plug with worn pellets, and the function of the converter deteriorates. The conversion efficiency of the unit declines and backpressure increases over the life of the converter.
Current catalytic converter designs have failed to hold the pellets reliably in tension and prevent fluidization in the pellet bed. One example of such failure is the well-known dual bed pellet converter, the upper bed of which often has extreme fluidization. This design has been dropped. The multi-million dollar recall programs in the auto industry attest to the inability of current designs to completely overcome catalytic converter problems. Such problems remain unsolved.
Another continuing concern with catalytic converters is the fact that excessive backpressure reduces engine efficiency and performance. Reducing backpressure without harming emission control is a continuing industry goal.
Yet another concern is the degree of unacceptable emissions during the start-up phase of engine operation, due to slow "light-up."Faster light-up is desirable.
There is a long-standing need for improved practical catalytic converters for the automotive industry.